Automatic leveling circuit for a composite video signal

Abstract

An automatic leveling circuit for a composite video signal is disclosed for use in a modem in a microwave communications system for adjusting the level of the signal at the transmitter while maintaining studio quality. The circuit employs an operational amplifier as the variable gain element and employs a light emitting diode photo-conductor pair in association with the amplifier to control the amplifier gain. In particular, the horizontal synchronizing pulse from the composite video signal is separated and used with a keyed clamp to establish the video at a desired dc reference value and is applied to a sample and hold circuit where the sampled pulse amplitude is held for comparison to an operator adjusted value. The output of the comparator is then used to control the light produced by the light emitting diode in a degenerative sense to hold the video level at the operator adjusted value. The range of level adjustment is typically 6db, and differential gain and differential phase are maintained to studio standards.

What is claimed is: 
     
       1. A circuit for automatically leveling to a desired amplitude level a composite video signal having horizontal synchronizing pulses and a video portion whose amplitude level bears a standard relationship to the amplitude level of the horizontal synchronizing pulses, comprising: A. means for supplying the composite video signal at an amplitude level typically differing from the desired amplitude level,   B. a wideband variable gain amplifier, for producing a composite video output signal, comprising an operational amplifier having a non-inverting terminal coupled to the composite video signal, an inverting terminal, and an output terminal and having a gain determined by the relationship of a first resistance (R f ), coupled between said output terminal and said inverting input terminal, and a second resistance (R g ) coupled between said inverting terminal and ground, said second resistance (R g ) being formed at least in part by a photo-sensitive resistor,   C. means coupled to the output terminal of said operational amplifier for developing an electrical quantity proportional to the amplitude level of the horizontal synchronizing pulses in the composite video output signal, and   D. a comparator, having first and second inputs, an adjustable reference amplitude level applied to the first input, and said electrical quantity proportional to the amplitude level of the horizontal synchronizing pulses applied to the second input, for producing an output dependent upon the difference between the amplitude levels of said first and second inputs, and   E. a light emitting diode, whose energization is controlled by the output of said comparator, optically coupled to said photo-sensitive resistor to vary the gain of the operational amplifier in a sense to maintain the amplitude level of the composite video output signal in substantial equality with said adjustable reference amplitude level, while maintaining accurate differential phase and differential gain.   
     
     
       2. The automatic leveling circuit set forth in claim 1, wherein said composite video signal may have color bursts and has a blanking level that represents one extremity of said horizontal synchronizing pulses and wherein said means for developing an electrical quantity proportional to the amplitude level of the horizontal synchronizing pulses comprises: (1) keyed clamping means coupled to the output terminal of said operational amplifier, for clamping said composite video output signal during the blanking level to an amplitude reference level to produce a clamped composite video signal and   (2) means for sampling said clamped composite video signal during the horizontal synchronizing pulses to produce potential difference samples proportional to the amplitude level of said horizontal synchronizing pulses.   
     
     
       3. The automatic leveling circuit set forth in claim 2 wherein said means for sampling said clamped composite video signal is a sample and hold circuit for producing a sustained voltage level equal to the amplitude level of said potential difference samples. 
     
     
       4. The automatic leveling circuit set forth in claim 3 further comprising: (i) threshold means coupled to said output terminal of said operational amplifier for stripping said horizontal synchronizing pulses from said composite video output signal,   (ii) first means, coupled to said threshold means, for deriving narrowed and delayed pulses coincident in time with the blanking level of said composite video output signal but avoiding the color bursts, if present, for timing said keyed clamping means and   (iii) second means, coupled to said threshold means, for deriving narrowed and delayed pulses lying within said horizontal synchronizing pulses for timing said sample and hold circuit.   
     
     
       5. The automatic leveling circuit set forth in claim 4 wherein said first means for deriving narrowed and delayed pulses comprises two successive one-shot multivibrators and said second means for deriving narrowed and delayed pulses comprises a single one-shot multivibrator.